Conventionally, various sensors have been devised as sensors used for a touch-type input device capable of detecting a pressing force (push amount) at the time of operation by a finger or the like. For example, Patent Document 1 (identified below) discloses a push amount detecting sensor including a piezoelectric sensor that outputs a push signal corresponding to a displacement amount obtained when an operation surface is pushed. Moreover, a push amount calculating unit is provided that integrates a difference between the push signal and a reference potential, and outputs a push amount detection signal based on an integration result.
Patent Document 2 (identified below) discloses a sensor that detects a pressing state according to the magnitude of a pressing force based on a signal generated by a piezoelectric element.
Patent Document 1: International Publication No. 2014/050683.
Patent Document 2: Japanese Patent Application Laid-Open No. H5-253037.
To use the piezoelectric sensor as a pressing sensor, it is necessary to reproduce a pressing profile by integrating the signal generated by the piezoelectric element. However, an integration error occurs due to a temperature drift of a circuit, an AD conversion error, plastic deformation of an adhesive provided in a sensor, and the like. Thus, as time elapses, an integrated value tends to deviate from an actual pressing force.
In the sensor disclosed in Patent Document 1, since the sensor is configured to detect a state where the operation surface is not touched and to reset the integrated value, the problem of the deviation can be avoided. However, since a contact detection sensor that detects a contact with the operation surface is required, the structure of the sensor is complicated. In addition, when signal processing for detecting the pressing force and signal processing by the contact detection sensor are individually performed, new processing for communicating a state showing whether or not the operation surface is touched by means of the contact detection sensor is required, so that an arithmetic processing load increases.